The invention concerns a screen, specifically a screen basket for sorting fiber suspensions. Known from U.S. Pat. No. 4,717,471 is a screen where the entrance area of the perforations is heavily flared in the shape of a funnel, the opening of the funnel having a very large angle, of more than 120 degrees, and perforations being round holes or orifices.
There are also screens formed by a parallel arrangement of rods, which form the screen duct or leave appropriate voids between them (DE 3,327,422).
A similar screen is known from U.S. Pat. No. 4,193,503, where the outlet side of the screen element is rounded in a convex fashion. But this screen type can only be used in conjunction with a virtually ribbed screen surface where the ribbing extends transverse to the major direction of flow along the screen surface. This arrangement imparts a wavy movement to the fiber suspension to concentrate the fibers and other more lightweight materials in an upper layer, i.e., at a greater distance from the screen surface, thus preventing these lightweight substances from passing through the screen perforations. This effect is contrary to the effect desired by the present invention, which is to sort the fibers--notably paper or cellulose fibers--out through the screen perforations, but prevent particles having dimensions greater than the desired fibers from passing through the screen surface.
The screen of DE 3,327,422 is susceptible to cloggings on the screen edges formed by the tilted rods. With such screens it is known that the throughput and the sorting integrity decrease with the sharpness of the profile while the susceptibility to clogging, for instance to cellulose buildup, increases.
The problem underlying the present invention is to reduce the susceptibility to clogging and increase sorting integrity during the sorting operation.